RT8063 View Datasheet(PDF) - Richtek Technology
Part Name
Description
Manufacturer
RT8063 Datasheet PDF : 11 Pages
| |||
RT8063
Application Information
The basic IC application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by CIN and COUT.
Main Control Loop
During normal operation, the internal high side power
switch (P-MOSFET) is turned on at the beginning of each
clock cycle. The inductor current increases until it reaches
the value defined by the output voltage (VCOMP) of the error
amplifier. The error amplifier adjusts its output voltage by
comparing the feedback signal from a resistive voltage
divider on the FB pin with an internal 0.8V reference. When
the load current increases, it causes a reduction in the
feedback voltage relative to the reference. The error amplifier
increases its output voltage until the average inductor
current matches the new load current. When the high
side power MOSFET shuts off, the synchronous power
switch (N-MOSFET) turns on until the beginning of the
next clock cycle.
Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT
= VREF × ⎛⎜⎝1 +
R1
R2
⎞⎟⎠
where VREF is 0.8V typical. The resistive voltage divider
allows the FB pin to sense a fraction of the output voltage
as shown in Figure 1.
VOUT
R1
FB
RT8063
R2
GND
Figure 1. Setting the Output Voltage
Soft-Start
The RT8063 includes an internal soft-start function that
gradually raises the clamp on the COMP pin.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values,
but at the expense of efficiency. On the other hand,
operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses, but
requires larger inductance and/or capacitance to maintain
low output ripple voltage.
The operating frequency of the IC is determined by an
external resistor, ROSC, that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator. The practical switching
frequency ranges from 200kHz to 2MHz. However, when
the RT pin is floating, the internal frequency is set at 2MHz.
Determine the RT resistor value by examining the curve
below. Please notice the minimum on time is about 90ns.
2.4
2.0
1.6
1.2
0.8
0.4
0.0
0 300 600 900 1200 1500 1800
RRT (k Ω)
Figure 2. Switching Frequency vs. RRT
2100
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current, DIL, increases with higher VIN and decreases
with higher inductance :
ΔIL
=
⎡
⎢⎣
VOUT
fxL
⎤
⎥⎦
× ⎛⎝⎜1
-
VOUT
VIN
⎞⎠⎟
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
is a registered trademark of Richtek Technology Corporation.
DS8063-07 November 2012
Share Link: 